“I heard a scream and turned to see a woman fall to the ground,” he told The Star. “I contacted the city centre CCTV unit on my radio and they called the emergency services.

“Within five or six minutes an ambulance and police were here.”

A police photographer took pictures inside and outside the taped-off First single-decker in the aftermath of the incident.

As traffic was halted, buses that had not yet crossed the junction with Leopold Street were diverted along Trippet Lane and Broad Lane.

The Supertram system was suspended in the city centre, with services heading only east and south from the Cathedral and north-west from Shalesmoor.

A limited single line service shuttled passengers from Shalesmoor to the University tram stop.

Bystander Mark Chadwick, 29, was waiting for a bus to Fulwood when the collision happened.

“I didn’t see the bus hit her but I saw her afterwards,” he said. “She was on the side of the road, just at the tram stop outside Tesco. It was as if she had fallen off the pavement. She looked conscious - she had an expression of pain on her face - but she was completely still. Two ambulances and three police cars arrived almost straightaway.”

Brandon Jones, deputy managing director of First South Yorkshire, said: “We dispatched our local management team to the scene to support the police and at this time we don’t have any details on the cause of the collision.

“Really we are helping the police, and we hope the young lady is okay. We don’t know exactly what her condition is yet. As we understand it, she was a pedestrian who was walking in the same direction as the bus and crossed in front of it.

“We need to fully investigate the incident with the police before we can draw any conclusions.”

A Yorkshire Ambulance Service spokeswoman confirmed a female casualty had been taken to the Northern General Hospital after a collision with a bus.

Neither she nor a police spokesman were able to confirm the extent of her injuries last night.